1. Field of the Invention
The present invention relates to audio speakers, and more particularly to tuning speakers.
2. State of the Art
In the manufacturing process of speakers it is desirable to build a speaker system, having a uniform and predictable input/output (I/O) response characteristic or I/O transfer function. Ideally, the analog audio signal coupled to the input of a speaker is what is provided at the ear of the listener. In reality, the audio signal that reaches the listener""s ear is the original audio signal plus some distortion caused by the speaker itself (e.g., its construction and the interaction of the components within it) and by the listening environment (e.g., the location of the listener, the acoustic characteristics of the room, etc) in which the audio signal must travel to reach the listener""s ear. This distortion can be represented as shown in FIG. 1A in which the relationship of the input signal to the speaker and the output signal of the speaker is defined by a first transfer function T1 and the relationship of the output signal from the speaker and the signal that reaches the ear of the listener is defined by a second transfer function T2. The first transfer function represents the distortion contributed by the speaker and the second transfer function represents the distortion contributed by the listening environment.
Currently, there are many techniques performed during the manufacture of the speaker to minimize the distortion caused by the speaker itself so as to provide the desired speaker response. FIG. 1B shows a simplified block diagram of a typical speaker 10 which includes a cabinet 11, a cross-over network 12, a set of amplifiers (Amp 13), and a set of transducers 14. An audio input signal is coupled to a cross-over network through a cabinet port. The cross-over network functions to break-up the frequency energy into several high, middle, and low frequency components and divert those frequency components to corresponding amplifiers and transducers. For instance, low-frequency components are coupled to big transducers (also referred to as woofers), the medium frequency components are coupled to the mid-range transducers, and the high frequency components are coupled to the small transducers (also referred to as tweeters). The transducers fit into ports 14 within the cabinet and output an audible analog signal through the ports, often through a mesh screen. Hence, there are four primary independent manufacturing variables (i.e., cabinet, cross-over network, amplifiers, and transducers) that must be dealt with on a speaker-by-speaker (or lot-by-lot) basis to manufacture a reproducible speaker.
Currently, the techniques used to tune a speaker such as shown in FIG. 1B are all mechanical, generally intrusive, and time-intensive since they are often performed by hand. For instance, one manner in which to tune a speaker""s response is to adjust potentiometers within the cabinet so as to tune the cross-over network. The cross-over network is tuned to adjust the manner in which the frequency ranges are diverted to each transducer and to reduce the bleeding of frequency ranges into each other. Since these potentiometers often reside within the cabinet, this technique is relatively intrusive requiring hand-tuning while the speaker is disassembled. In addition, components, such a large inductors, within the crossover network might be physically moved to tweak effects caused by magnetic flux.
Another way in which a speaker is tuned is to use holes within the cabinet to modify the resonance of the cabinet by enlarging the holes until the desired resonance is achieved. The bass reflex of the cabinet can also be tuned by placing different length tubes into a passive output port of the cabinet to affect cabinet resonance.
The manner in which speaker users adjust for the distortion caused by the listening environment is to 1) modify the environment to improve its acoustics, 2) manually adjust speaker output characteristics such as treble and bass settings, or 3) move the speaker and the listener with respect to each other to affect the angle(s) in which the audio signal is received by the listenerxe2x80x94all of these techniques being an inexact and cumbersome tuning technique.
The present invention is a reproducible, non-intrusive system and method of tuning a speaker which does not require independent physical tuning of each of the physical manufacturing variables of the cabinet or inexact tuning of the listening environment.
A programmable speaker and a system and method of tuning the speaker uses digital signal processing and stored characterization data to obtain the desired transfer function for the speaker. The programmable speaker includes a programmable portion having a processing portion and a memory portion for storing characterization data. The processing portion receives an input audio signal. The characterization data stored in the memory portion is accessed by the processing portion to perform a transform function on the input signal to generate a transformed signal which compensates for the distortion of the input signal resulting from the physical elements of the speaker and from the listening environment. As a result, each physical speaker element does not require individual tuning and the listening environment need not be altered and instead an overall distortion compensation is achieved by performing the transform function on the input audio signal. The transformed signal is coupled to the output portion of the speaker which produces an audible analog output signal representing the input signal compensated with the transform function according to the characterization data. In one embodiment, the characterization data is the weighting coefficients of the transform function.
A system for tuning the programmable speaker includes a microphone for receiving the audible output signal produced by the speaker and feeding it back to a tuning device. The tuning device includes a reference signal generator for providing a reference signal to the processing portion of the programmable speaker. The tuning device performs a comparison analysis between the audible output signal and the input reference signal and generates a control signal including updated characterization data dependent on the comparison. The control signals are coupled to the programmable input portion of the speaker, are stored in the memory portion, and are used again to tune the speaker by performing the transform function on the input reference signal. The characterization data is used by the processing portion to minimize the distortion by making the input and the output audio signals detected by the microphone as similar as possible. This cycle of providing updated control signals, transforming a reference signal using the updated control signals to generate an output signal, feeding back the output signal, and analyzing the signal to generate a new updated control signal is performed until the reference signal and the signal detected by the microphone match and/or exhibit the desired transfer function relationship.
In one embodiment, the reference signal is chosen so as to tune the speaker to have a given overall operational characteristic such as having a stronger bass (lower frequencies) or alternatively, a strong mid-range (mid-frequencies). In another embodiment, more than one reference signal may be used to tune the speaker to give the speaker a variety of operational characteristics.
In another embodiment, the processing portion performs a cross-over type transfer function so as to generate a plurality of digital signals each corresponding to a different frequency range to be diverted to a different output transducer of the speaker.
In still another embodiment, the processing portion is implemented with a digital signal processing (DSP) unit and an associated DSP memory system. The DSP portion processes the input reference signal according to the characterization data accessed from a non-volatile memory. In another embodiment, the processing portion includes function specific hardware accelerator circuitry to perform mathematical operations used to implement the transform function such as addition and multiplication operations of signals so as to minimize overall processing time of the audio input signal.
In still another embodiment, the output drive portion includes a plurality of digital-to-analog converters for receiving the plurality of transformed signals generated by the cross-over transform function from the programmable portion and for converting them into a plurality of analog signals. The converted signals are coupled to an amplifier stage. The amplified signals are then coupled to the speaker transducers for outputting an audible signal corresponding to the transformed input signal.
In still another embodiment, the speaker is first pre-programmed during a manufacturing tuning process to compensate for distortion caused by the individual speaker elements wherein a set of coefficients are pre-programmed into the memory portion. The pre-programmed speaker is then programmed for a second time by the consumer to tune the speaker to compensate for distortion caused by a specific listening environment. In this embodiment, the second tuning process is implemented by placing the microphone in a selected location within the listening environment. For instance, the microphone may be placed at the location which the listener is to be seated. The tuning portion couples a reference input signal to the speaker and the speaker processing portion transforms the reference signal using the manufacturers pre-programmed coefficients to generate an output reference signal. The microphone receives the output reference signal from the speaker along with distortion resulting from the audio characteristics of the listening environment. The tuning means then adjusts the set of coefficients to compensate for the distortion caused by the listening environment. The reference signal is then again transformed by the processing portion using the set of coefficients adjusted for the particular listening environment. The speaker can perform subsequent tuning cycles until the speaker is tuned to the environment as established by the selected position of the microphone. In accordance with this embodiment, a plurality of sets of coefficients can then be stored in the memory portionxe2x80x94each set corresponding to a different listening environment. In this way the consumer can subsequently retrieve the stored coefficients depending on a desired listening environment.